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How to Sum Elements of Two Lists in Python

renegadecoder94 profile image Jeremy Grifski Originally published at therenegadecoder.com Updated on ・4 min read

Welcome back to another edition of the How to Python series. This time I want to sum elements of two lists in Python. I got the inspiration for this topic while trying to do just this at work, so let’s see how it goes!

Problem Introduction

Recently, I ran into a problem where a library wasn’t working exactly how I wanted, so I had to hack together the results to make my life a bit easier. In this scenario, I had a connection library which I was using to list all the available devices. However, the list functionality short circuited for certain types of connections, so it never actually listed everything. As a result, I was forced to run the function twice: once for USB and again for Ethernet.

The results of this list function returned a list that looked something like the following:

# [size, [types], [interfaces], [serial numbers], [IP addresses]]
[2, [7, 7], [1, 2], [2314567, 8374163], [0, 84302738]]

However, I was stuck retrieving the lists separately:

[1, [7], [2], [8374163], [84302738]]  # Ethernet
[1, [7], [1], [2314567], [0]]  # USB

Naturally, I wanted to be able to merge the two lists back into what I was expecting initially. However, I wasn’t totally sure how I was going to do that. So, let’s take a look at some possible solutions.

Solutions

When it comes to merging two lists, there are several possible ways to do it. Here are just a few.

Merge Two Lists by Hand

At first, we might try merging the two lists by hand. In other words, we might try the following:

ethernet_devices = [1, [7], [2], [8374163], [84302738]]
usb_devices = [1, [7], [1], [2314567], [0]]
all_devices = [
    ethernet_devices[0] + usb_devices[0],
    ethernet_devices[1] + usb_devices[1],
    ethernet_devices[2] + usb_devices[2],
    ethernet_devices[3] + usb_devices[3],
    ethernet_devices[4] + usb_devices[4]
]

Now, that solution is hardly elegant, but it gets the job done. However, there has to be a better way. After all, we are already taking advantage of the fact that lists can be merged using the same operator as addition. Why not take advantage of this property in a loop?

Sum Elements of Two Lists with a Comprehension

Just like the last lesson on inverting dictionaries in Python, we can take advantage of comprehensions to dramatically simplify this problem. Let’s take a look:

ethernet_devices = [1, [7], [2], [8374163], [84302738]]
usb_devices = [1, [7], [1], [2314567], [0]]
all_devices = [x + y for x, y in zip(ethernet_devices, usb_devices)]

Now we’re talking! That’s five lines of tedious mapping compressed down into a simple list comprehension. But wait, it gets better:

all_devices = [sum(pair) for pair in zip(ethernet_devices, usb_devices)]

Or, does it? Thanks to our fried, rhymes, we'll notice that this solution doesn't actually work for our situation. While it does a great job summing integers in an iterable, it crashes when trying to merge two sublists with the following error:

Traceback (most recent call last):
  File "<pyshell#3>", line 1, in <module>
    all_devices = [sum(pair) for pair in zip(ethernet_devices, usb_devices)]
  File "<pyshell#3>", line 1, in <listcomp>
    all_devices = [sum(pair) for pair in zip(ethernet_devices, usb_devices)]
TypeError: unsupported operand type(s) for +: 'int' and 'list'

That said, you may find it useful for merging two lists of strictly integers.

Sum Elements of Two Lists with a Mapping

At this point, we’ve basically answered our question. However, there is another solution which requires arguably even less code. Check it out:

ethernet_devices = [1, [7], [2], [8374163], [84302738]]
usb_devices = [1, [7], [1], [2314567], [0]]
import operator
all_devices = list(map(operator.add, ethernet_devices, usb_devices))

Not totally certain of the performance impact here, but it sure is a pretty solution. Of course, we also need to add a dependency which does make this solution a bit less attractive. In fact, if we were going to go down that road, we could easily leverage numpy:

ethernet_devices = [1, [7], [2], [8374163], [84302738]]
usb_devices = [1, [7], [1], [2314567], [0]]
import numpy as np
all_devices = np.add(ethernet_devices, usb_devices)

However, numpy is a massive library, so we should probably stick with our list comprehensions.

A Little Recap

Using the handful of methods above, we should be able to sum elements of two lists.

ethernet_devices = [1, [7], [2], [8374163], [84302738]]
usb_devices = [1, [7], [1], [2314567], [0]]

# The long way
all_devices = [
    ethernet_devices[0] + usb_devices[0],
    ethernet_devices[1] + usb_devices[1],
    ethernet_devices[2] + usb_devices[2],
    ethernet_devices[3] + usb_devices[3],
    ethernet_devices[4] + usb_devices[4]
]

# Some comprehension magic
all_devices = [x + y for x, y in zip(ethernet_devices, usb_devices)]

# Let's use maps
import operator 
all_devices = list(map(operator.add, ethernet_devices, usb_devices))

# We can't forget our favorite computation library
import numpy as np 
all_devices = np.add(ethernet_devices, usb_devices)

As we can see, there are a lot of ways to run an element-wise sum of two lists. Take your pick.

As always, thanks for stopping by! If you’re interested in learning more about Python, consider subscribing to The Renegade Coder, so you’ll never miss another article.

Discussion

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rhymes

Maybe you can add benchmarks to the examples, to have an idea about costs.

There's an example that won't work:

all_devices = [sum(pair) for pair in zip(ethernet_devices, usb_devices)]

this does not actually work:

TypeError: unsupported operand type(s) for +: 'int' and 'list'

The reason is the first item in your lists is an integer, not a list. The whole thing works in the manual examples because Python's + operator has meaning for two integers and for two lists.

Let's see what you're asking for a result:

[2, [7, 7], [2, 1], [8374163, 2314567], [84302738, 0]]

And this is the intermediate iterable generated by zip:

>>> [pair for pair in zip(ethernet_devices, usb_devices)]
[(1, 1), ([7], [7]), ([2], [1]), ([8374163], [2314567]), ([84302738], [0])]

Sum though works only on a list of numbers, so it will break on the second tuple. You can quickly see it by executing:

>>> sum(([7], [7]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'list'

The third example works because operator.add it's just the functional version of the + operator, which does:

>>> (1).__add__(1)
2
>>> ([7]).__add__([7])
[7, 7]

Hope this helps :)

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renegadecoder94 profile image
Jeremy Grifski Author

Good catch! I must have made a mistake when I was copying this article over. I’ll correct that section when I get a chance.

EDIT: After a closer look, I guess I just never tested that solution. Whoops! I referenced your great comment in the original post as well.